Ultrasonic flow measuring devices are applied often in process and automation technology. They make possible contactless determination of the volume and/or mass flow rate of a medium in a pipeline.
Known ultrasonic flow measuring devices work either by the Doppler principle or the travel-time-difference principle. In the case of the travel-time-difference principle, the different travel times of the ultrasonic measuring signals in the direction of flow, and counter to the direction of flow, of the medium are exploited. To this end, the ultrasonic measuring signals are alternatingly issued, respectively received, in the direction of flow, and counter to the direction of flow, of the medium. On the basis of the travel-time-difference of the ultrasonic measuring signals, the flow velocity can be determined, and, with that and known diameter of the pipe, the volume flow rate of the medium, or, with known density, the mass flow rate of the medium.
In the case of the Doppler principle, ultrasonic measuring signals of predetermined frequency are coupled into the flowing medium. The ultrasonic measuring signals reflected in the medium are evaluated. On the basis of a frequency shift occurring between the ultrasonic measuring signal which was coupled into the medium and the reflected ultrasonic measuring signal, likewise the flow velocity of the medium, or the volume and/or mass flow rate, can be determined.
The use of flow measuring devices working according to the Doppler principle is only possible, when present in the medium are air bubbles or impurities, on which the ultrasonic measuring signals are reflected. Thus, use of ultrasonic flow measuring devices using the Doppler principle is rather limited, compared to ultrasonic flow measuring devices using the travel-time-difference principle.
With respect to types of measuring devices, a distinction is drawn between ultrasonic flow measuring pickups, which are inserted into the pipeline, and clamp-on flow measuring devices, where the ultrasonic transducers are pressed onto the pipeline externally by means of a clamp connection. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
In the case of the two types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated at a predetermined angle into, and/or received from, the pipeline, or measuring tube, as the case may be, containing the flowing medium. In order to achieve an optimum impedance matching, the ultrasonic measuring signals are coupled into, or out of, the pipeline via a lead-in member, or a coupling wedge, as the case may be. Principal component of an ultrasonic transducer is at least one piezoelectric element, which produces and/or receives the ultrasonic measuring signals.
The ultrasonic measuring signals produced in a piezoelectric element are led via the coupling wedge, or lead-in member, as the case may be, and, in the case of a clamp-on flow measuring device, through the pipe wall, into the liquid medium. Since the velocities of sound in a liquid and in plastic differ from one another, the ultrasonic waves are refracted at the transition from one medium into the other. The angle of refraction at the transition from one medium into another medium is dependent on the ratio of the velocities of sound cm, cn in the two media n, m.
Mathematically, Snell's law can preferably be expressed according to the following formula:cn/sin αn=cm/sin αm=const.  (1)where:cn is the velocity of sound e.g. in the coupling wedge made, for example, of plastic;cm is the velocity of sound e.g. in the medium, which is, for example, water;αn is the angle between the sound path and the normal to the bounding surface of the coupling wedge at the point where the ultrasonic measuring signal passes through the bounding surface; andαm is the angle between the sound path and the normal to the bounding surface of the medium at the point where the ultrasonic measuring signal passes through the bounding surface.
With coupling wedges, or lead-in members, of plastic, among other things, a good impedance matching can be achieved; however, the velocity of sound in plastic has a relatively strong temperature dependence. Typically, the velocity of sound in plastic changes from about 2500 m/s at 25° C. to about 2200 m/s at 130° C. In addition to the change of travel time of ultrasonic measuring signals in the plastic of the coupling wedge brought about by temperature, the direction of propagation of the ultrasonic measuring signals in the flowing medium also changes. Both changes, in the case of an ultrasonic flow measuring device operating according to the travel time difference method, naturally act unfavorably on the accuracy of measurement. Added to this is the fact that the propagation velocity exhibits, in certain media, likewise a strong temperature dependence.
For coping with the temperature dependence of the coupling wedges, it is known from WO 02/39069 A2 to construct the coupling element out of a plurality of segments in the form of circular arcs. Preferably, the segments are made of metal. The individual segments are arranged separated from one another and they extend from a contact plane, which faces the piezoelectric element, out to a base plate, which is connected with the pipe wall. The length of the individual segments is, in such case, so measured, that the ultrasonic measuring signals are radiated and received at a predetermined angle at the base plate. This embodiment is, however, relatively complex.